Hi, Kireeti, Aldrin The MPLS label was originally meant to identify a link, and got overridden in rfc4364 as a _local_ identifier of VPNs and has now further morphed in draft-drake-nvo3-evpn-control-plane to be a _global_ identifier of VPNs to accommodate the VNID. As a semantic conservative, this drift worries me. :-) The RD is meant to distinguish between routes, just like the VNID is meant to distinguish between routes from different tenants. If using a type 0 bothers RD purists, maybe we could define a new type for it? The VNID is typically administratively assigned, just like the RD. It's true that the RD has never been seen before on the wire, but the VNID has already broken that taboo. It's time to call the VNID what it really is - an RD.
Semantic arguments aside, the chief reason I preferred using the RD instead of the MPLS label is that it would allow us to encode the complete 24 bit VNID in the BGP route, instead of truncating it to 20 bits to fit inside the MPLS label field. -- Sunny From: Kireeti Kompella <[email protected]> To: Aldrin Isaac <[email protected]>, Cc: Sunny Rajagopalan/Santa Clara/IBM@IBMUS, "[email protected]" <[email protected]>, "[email protected]" <[email protected]>, "[email protected]" <[email protected]>, "[email protected]" <[email protected]>, "[email protected]" <[email protected]>, "[email protected]" <[email protected]>, "[email protected]" <[email protected]>, "[email protected]" <[email protected]>, "[email protected]" <[email protected]> Date: 09/26/2012 09:39 AM Subject: Re: [nvo3] comments on draft-drake-nvo3-evpn-control-plane Thanks, Aldrin! More inline. On Sep 25, 2012, at 21:23, Aldrin Isaac <[email protected]> wrote: >> 1) I would suggest *not* altering the semantics of the MPLS label in the BGP >> route. Note that no one is suggesting altering the semantics of an MPLS label. What's proposed is altering the interpretation of a three-byte field in the NLRI. >> Instead, use the route distinguisher to carry the 24-bit VNID (this >> is arguably better since the semantics of the RD align better with the >> semantics of the VNID). I would suggest encoding this as a type 0 RD, with >> the VNID going into the Assigned number sub-field. In addition, call out >> that an MPLS label value of 0 in the BGP route is a valid value, and will be >> used by PEs which do not support MPLS encap. > > The RD was not intended to be used to signal data plane bits. That's > what the label field is for. Exactly. Worse than that, the RD is responsible for _distinguishing_ routes. If VNIDs are locally generated (an option well worth holding on to), all manner of hell will break loose. Kireeti
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